This invention relates to a remote instrumentation system, for use with equipment providing a three phase power supply to a motor, comprising signalling means, including a transducer, for connection between a neutral point of the motor winding circuit and the motor chassis, and sensing means for connection to the three phase power supply circuit at a point remote from said motor, said sensing means being arranged to provide a DC signal to said signalling means via said motor winding circuit and to detect a transducer measurement by monitoring the DC signal passed by said signalling means.
In the field of remote instrumentation it is often desirable to provide power to the remote instrumentation through an electrical conductor and to receive and transmit signals over the same conductor.
Such a situation arises in the oil industry, for example, where instrumentation at the bottom of an oil well is powered by, and communicates with, surface equipment. To minimize the cost of the interconnecting cable, the remote instrumentation is often powered by a DC signal on a single conductor cable and the signal is returned as an AC frequency, or pulse train, on the same conductor.
In some installations, an electrical submersible pump is positioned at the bottom of the oil well which is powered from the surface by AC current, typically at the normal mains frequency of 50 or 60 Hertz. In these cases it is most convenient to transmit any instrumentation signals from the locality of the pump to the surface via the power cable or cables, rather than by installing a separate cable for these signals.
It is known that high frequency signals can be imposed on the power lines. These frequencies can later be separated out from the mains frequency using filters to recover the signal information at the surface. However, these high frequency signals cannot pass through the motor windings of the submersible pump, and so a cable splice into the power cable above the pump is required. This is highly undesirable, as this splice (or junction) is a cause of unreliability in the aggressive environment found at the bottom of an oil well. In addition, any failure in the instrumentation can potentially cause a low impedance path for the electrical pump power, and so prevent the pump from operating.
It is also known that a variable resistance transducer (often referred to as a potentiometric transducer) may be used to communicate pressure or temperature information over the power cables of a submersible pump. Submersible pumps generally employ three-phase motors, and at the bottom of such a motor, the three phases are connected to form a "star" or neutral point. The potentiometric transducer may be connected between this star point and the motor chassis.
The surface equipment may measure the resistance of this transducer via the power cable and motor windings. The advantage of this well known system is that no high-voltage cable splices are required, and in addition, any failure of the transducer system will not prevent continued motor operation, as the star point may be shorted to chassis, or left open, with no adverse effect on motor operation. One disadvantage of this system arises from the resistance of the power cable conductors that are electrically in series with the potentiometric transducer. Any change in this power cable resistance will affect the ultimate reading. Furthermore, this technique requires the use of potentiometric transducers which are unreliable and inaccurate.
The first disadvantage may be reduced to a certain extent by using diodes to steer the measuring current through the transducer when powered from the surface using one electrical polarity, and to short out the transducer when powered using the converse polarity. In this way, the first polarity provides the sum of transducer and cable resistance, and the second polarity provides just the cable resistance. Hence the true transducer resistance may be calculated. However, the other above-mentioned disadvantages remain; and furthermore, no more than one transducer may be used in this system -- or two, if the cable resistance correction feature is not used.
It is an object of the invention to overcome, or at least reduce, at least one of the above-mentioned disadvantages.